This invention relates to a data reader and methods of reading data. It is particularly applicable to data storage devices, but has wider applicability.
This invention will be described in detail in relation to magnetic tape data storage systems, although it has wider applicability and may be applied to any number of different types of data receiving system.
In any form of data storage system, data is stored on a physical medium. A reading mechanism is provided with which data can be retrieved from the physical medium. In storage systems such as magnetic tape data storage system the physical medium can undergo random, unpredictable, fluctuations. In the case of magnetic tape drives the physical separation of the tape and the read head can vary significantly and so provide the random fluctuations. The random fluctuations can attenuate the strength of the signal being read from the physical medium. In general, such attenuation must be accounted for before data read from the medium can be successfully decoded.
Another example of such random fluctuations is in the field of hard disk drives in which the height of the read head above the disc platter can vary, and cause similar fluctuations. However, fluctuations of this kind tend to be far less pronounced and significant than in magnetic tape data storage systems. Such fluctuations could also occur due to variations in the quality of the storage medium.
Means for compensating for the attenuation is well known. In some embodiments the means comprises an automatic gain control (AGC), which adjusts the amplification applied to the signal produced by the read head to compensate for the attenuation arising from the random fluctuations. However, some prior arrangements of AGC have not operated as effectively as may be desired.
It is known to use such AGC""s to control a variable gain amplifier such that the signal is amplified appropriately to account for the fluctuations and an example of a known arrangement is shown in FIG. 1. It is known to provide analogue automatic gain controls, which are fast but do not control the gain of the amplifier as accurately as is desired. It is also known to provide digital automatic gain controls, which due to the digital filtering, and processing required before a signal can be input thereto, are slow (and therefore, cannot respond to rapid changes), although they allow the gain of the amplifier to be accurately controlled.
It is also known to provide both an analogue and a digital gain control in independent loops, but such implementations have been difficult to control, and are complicated to fabricate.
In U.S. Pat. No. 5,375,145, which relates specifically to bard disk drive systems, both digital and analogue variable gain control feedback loops are provided and feed back their respective signals to plural variable gain amplifier stages. According to this patent, the analogue gain control loop is active to vary gain control only during a non-reading mode of operation. During a reading mode, the analogue gain control loop is static (i.e. its gain control is fixed and does not vary) and the digital gain control feedback loop is active to provide fine adjustments to the xe2x80x98set-pointxe2x80x99 of the analogue gain control feedback loop.
It is an object of the present invention to provide a data reader and method of reading data that can accommodate random fluctuations, of the kind particularly exhibited by magnetic tape data storage systems.
According to a first aspect of the invention there is provided a data reader arranged to produce an output signal on reading a data signal, comprising processing circuitry being arranged to produce said output signal, said processing circuitry including at least a first and a second feedback loop, each of the feedback loops being arranged to produce a feedback signal that varies in response to variations in the data signal, said first feedback loop having a first characteristic and said second feedback loop having a second characteristic, the processing circuitry being arranged to apply more weight to the varying feedback signal of the feedback loop having the appropriate characteristic to control processing of said data signal at a particular instance.
An advantage of such a device is that the different characteristics of the feedback loops of the processing circuit can be employed to help ensure that the data signal is decoded accurately.
The data signal may be produced by the reading of a data-holding medium, or by the reception of a signal over a data transmission medium. The data-holding medium may be any one of the following: a tape, a hard disk drive platter, a CDROM, a DVD-ROM, any other computer readable data carrier. The data transmission medium may be any one of the following: electromagnetic waves, infrared links, other optical links (An infra red link may correspond to the IRDA protocol).
An initial amplifier may be provided to perform initial conditioning of the data signal. Preferably, the initial amplifier is a variable gain amplifier. The processing circuitry is preferably arranged such that the feedback signals control the gain of the variable gain amplifier. Such an arrangement is advantageous because it provides a convenient structure, which can be used to initially condition the signal.
Conveniently, a summer is provided that sums the feedback signals produced by each of the feedback loops and produces a summed feedback signal. The variable gain amplifier may be arranged such that the summed feedback signal controls its gain.
At least one of the feedback loops includes a feedback signal producer arranged to produce the feedback signal of that feedback loop. Preferably, each of the feedback loops includes a feedback signal producer arranged to produce the feedback signal of that loop.
Conveniently, at least one of the feedback signal producers is an automatic gain control.
Conveniently, at least one of the feedback loops may be arranged so that the feedback signal producer therein has an analogue signal input thereto. Feedback signal producers working in the analogue domain have the advantage that they have a quick response time and therefore can rapidly respond to fluctuations in signals input thereto. However, it can be difficult to provide analogue signal producers that are as accurate as may be desired.
In some embodiments there may be provided a plurality of feedback loops having feedback signal producers therein arranged to have an analogue signal input thereto.
The or each analogue feedback signal producer may be arranged to produce the feedback signal by applying thresholds to an analogue signal input thereto. As the skilled person will appreciate the analogue feedback signal producer may be arranged to produce the feedback signal by detecting analogue signal amplitude or power in any other suitable manner.
Further, at least one of the feedback loops may be arranged such that the feedback signal producer therein has a digital signal input thereto. Feedback signal producers working in the digital domain tend to be more accurate (i.e. control the gain to a closer tolerance) than those working in the analogue domain. However, in the data reader of the current invention the digital signal input to the feedback signal producer may be produced using a digital filter. Such a digital filter will introduce a time lag into the digital signal and, therefore, the feedback signal producer may not respond to fluctuations in the signal from the data reader as quickly as may be desired. Further, AGC""s working in the analogue domain can be read-data-pattern dependent which is undesirable and it is therefore advantageous to use an AGC working in the digital domain.
In some embodiments there may be provided a plurality of feedback loops having feedback signal producers therein arranged to have a digital signal input thereto. Conveniently, an analogue to digital converter is provided to digitise the analogue signal produced by the initial amplifier. The processing circuitry may be arranged to pass the analogue signal produced by the initial amplifier to the analogue feedback signal producer. Further, the processing circuitry may be arranged to pass the digitised analogue signal to the digital feedback signal producer.
In some embodiments a digital amplifier may be provided, which is conveniently arranged to receive the signal from the analogue to digital converter.
Feedback loops may be provided and arranged to alter the gain of the digital amplifier.
Advantageously, the first feedback loop may be arranged to have a short time constant. Such an arrangement is advantageous because it can rapidly adjust the gain of the variable gain amplifier as appropriate.
The first feedback loop may be arranged to respond within roughly 10 bits of data to within 10000 data bits. The time constant will vary depending upon the application in which the data reader is being used. The time constant may be programmable. Any number of bits in between these limits may also be possible. For instance roughly any one of the following number of bits may be possible: 50, 100, 150, 250, 500, 1000, 2500, 5000, 7500, (or any number in between).
Further, the second feedback loop may be arranged to have a longer time constant than the first feedback loop. This is advantageous because it may mean that the produced feedback signal is more accurate.
At least one of the feedback loops comprises a scaler arranged to scale the feedback signal present in the feedback loop. Preferably, each of the feedback loops is provided with a scaler. Such scalers provide a convenient way of controlling which of the feedback loops carries the greatest weight to control processing of said signal. Preferably, the processing circuitry is arranged to alter the scale applied by the scalers to adjust the weight applied to each of the feedback signals.
The processing electronics may be arranged to alter the scalers, such that the value of one scaler affects the value of other scalers. In some embodiments the value of one scaler may alter the value of other scalers according to a known function.
A signal-processing block may be provided to produce an output signal from the data reader, and conveniently the signal-processing block has as its input the digitised analogue signal produced by the initial amplifier.
Preferably the processing circuitry is arranged to control the value of the scalers based upon the results of statistical analysis on the output signal.
The processing circuitry may be arranged to vary the value of the scalers during operation of the data reader. This varying may provide for different modes of operation, for example to provide a defect-handling mode.
According to a second aspect of the invention there is provided a data-receiving device incorporating a data reader according to the first aspect of the invention.
The data-receiving device may be any form of device arranged to receive a signal. For example the data-receiving device may be a telephone (may be a mobile telephone), a MODEM, a network card, a network router (or switch, hub, bridge, etc.), a printer, etc.
Alternatively, or additionally, the data-receiving device may be a data storage device.
In one embodiment the data storage device is arranged to receive magnetic tapes wherein the magnetic tape provides the data-holding medium. However, the device may be arranged to read data from a hard disk wherein the disk platter is the data-holding medium. The storage device may be arranged to read data from other forms of data-holding medium.
According to a third aspect of the invention there is provided a method of reading a data signal to produce an output signal comprising initially conditioning said data signal, the initial conditioning being controlled by at least two feedback loops of differing characteristics, the method comprising changing the weight applied to each of the feedback loops to make use of the appropriate characteristic.
At least one of the feedback loops may have a digital signal as an input and generate a feedback signal. Further, at least one of the feedback loops may have an analogue signal as an input and generate a feedback signal.
The initial conditioning of the signal may be to remove unwanted amplitude variations present on the signal produced on reading the data signal.
The method may apply a scaling factor to at least one of the feedback signals, and preferably, a scaling factor is applied to each feedback signals. Such scaling factors provide a convenient way of adjusting the weight given to the feedback loops.
Each of the feedback signals may be summed, and the summed feedback signal used to control the initial conditioning of the signal produced on reading the data signal.
The method may ensure that at least one of the feedback loops has a short time constant and can therefore respond rapidly, to fluctuations in the signal produced reading the data signal.
The method may further ensure that at least one of the feedback loops can accurately remove unwanted fluctuations in the signal produced on reading the data signal.